structured config #4: support for session and probes

[tuunit]

Description

Warning

This is a PR stacked on top of:

• introduce mapstructure decoder for yaml parsing #2628
• legacy file structure refactoring and cookie support #2629
• support for proxy options #2630

Actual stacked changes can be viewed here:
https://github.com/tuunit/oauth2-proxy/pull/9/files

1. Support for probe options
2. Support for session options
3. Refactoring name of templates to page templates
4. Support for page template options

How Has This Been Tested?

1. Test cases have been extend / adapted.
2. Starting the application with toml config file
3. Using the conversion function from toml to yaml
4. Starting the application with yaml config file

Checklist:

• My change requires a change to the documentation or CHANGELOG.
• I have updated the documentation/CHANGELOG accordingly.
• I have created a feature (non-master) branch for my PR.
• I have written tests for my code changes.